Update 23 – September 6, 2020

Amanda Gevens, Dept. Chair, Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, Cell: 608-575-3029.

Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations (Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, UW-Madison Plant Pathology). A P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application. A DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application. Red text in table indicates threshold has been met/surpassed. Weather data used in these calculations comes from weather stations that are placed in potato fields in each of the four locations. Data are available in multiple formats for each station at: https://vegpath.plantpath.wisc.edu/dsv/

Location Planting Date 50% Emergence


Disease Severity

Values 9/6/2020

Potato Physiological

Days 9/6/2020

Grand Marsh Early Apr 17 May 18 161 863
Mid Apr 25 May 26 158 808
Late May 6 June 1 155 767
Hancock Early Apr 8 May 18 78 843
Mid Apr 20 May 25 76 792
Late May 4 May 30 73 754
Plover Early Apr 10 May 23 136 804
Mid Apr 20 May 30 130 750
Late May 5 June 1 130 738
Antigo Early May 14 June 5 78 720
Mid May 24 June 10 78 683
Late Jun 1 June 17 77 635

Late Blight Management: Our DSVs are reported here from emergence to September 6. Over the past week, we saw low accumulations (just 5 to 11 DSVs over the course of a week; reminder, max potential DSV per day is 8 under optimum disease conditions). Plantings of potatoes in the Grand Marsh, Hancock, Plover, and Antigo areas should receive routine (~weekly) preventative fungicide application for late blight management. During senescence to harvest, tubers remain susceptible to late blight. In particular, for crops fated for long term storage, continued application of mancozeb-containing fungicide can provide management of the tuber phase of late blight which can happen during spore movement and ‘washout’ events. This late season fungicide treatment is especially important if fields are/were proximal to any known infections, and if the maturity of crops varies greatly within your area (due to risk of foliar late blight and spore availability).

Early Blight Management: PDays are exceeding the threshold of 300 for early planted potatoes in Grand Marsh, Hancock, Plover, and Antigo areas. For more information about fungicide selections, please see the Potato section of the A3422 Commercial Vegetable Production Guide for Wisconsin, 2020. https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3422-2020.pdf

National late blight update: No new reports of late blight in Wisconsin.  Across the US however, there were new reports: on Aug 20 US-23 was confirmed on tomato and potato in Chautauqua County in western NY state,  So far this season, there have been just two WI confirmations of late blight from tomato (Pierce Co.) and potato (Adams Co. US-23) this season. No widespread movement from these sites as far as I’m aware. Earlier this season, late blight had been reported in North Carolina (tomato US- 23), Washington, and it was found in British Columbia, western Canada (Delta and Surrey) over a month ago now. The site: https://usablight.org/map/ includes reports as they are submitted in the US. Previous reports documented the disease in AL, GA, NC, NY, FL, TN, WA, and WI. Where the late blight pathogen has been tested in the US so far this year, the clonal lineage has been US-23.

National cucurbit downy mildew update: No downy mildew reported from WI at this time through my Vegetable Pathology Lab or the UW Plant Disease Diagnostic Clinic. Reports to date, have come from: AL, CT, DE, GA, IL, IN, KS, KY, MA, MD, ME, MI, MS, NC, NH, NJ, NY, OH, Ontario & Quebec Canada, PA, SC, TN, VA, and WV. No forecasted movement of the pathogen in our direction, with prevailing air moving eastward. https://cdm.ipmpipe.org/forecasting/


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